Increased utilization of coal is of considerable present interest as one way to increase the supply of needed energy. Since burning of coal is mostly limited to large stationary facilities and is environmentally unacceptable due to high sulfur and ash levels, conversion of coal to produce a clean liquid has been undergoing an intensive research and development effort by private undertakings as well as under government sponsorship.
In direct coal conversion processes, such as the solvent refining of coal, the most difficult problem encountered in providing an acceptable liquefied coal product is the removal therefrom of the residual ash particulates and/or other contained solid particulates, such as finely divided insoluble carbonaceous particles, carried in the liquid product arising from the liquefaction process. Typically, the ash content of raw coal may range from about 5 to 12% by weight, depending upon the source of the coal. The particle size distribution of particulates from coal liquefaction reactors is not well defined but limited data indicate that the mass median particle diameter ranges from 3 to 15 microns.